Tuesday, June 30, 2015

Cool experiments with dry ice

Yesterday my wife and I did our latest kids science demo gig, at a holidays kid club at our church. My son has encouraged us to come up with some new demonstrations since some of the kids have already seen some of the old favourites, such as Elephants toothpaste, coke and mentos rocket, and a few others here.

At first I was pretty excited when I saw this Youtube video of an LED powered by a lemon. Watch it and see what you think. I even got some LEDs to tried and do it. At the end of the post I tell the rest of the story.

We settled on a few demos with dry ice [solid carbon dioxide]. The unique feature is that at atmospheric pressure the solid does not melt [become liquid] but sublimates [becomes vapour]. This is because in the phase diagram the pressure of the triple point [5 atm] is above atmospheric pressure.

Here are some of the demonstrations. Many of them rely on the simple fact that the volume of one gram of vapour is of the order of five hundred times larger than the volume of one gram of solid. A good exercise for high school and college students [and you!] is to come up with a simple "back of the envelope" argument as to why this is so.

A. Put a few pellets of dry ice in a zip lock bag and seal it.
After a few minutes the pressure build up due to sublimation causes the bag to "pop". I quite like this because the pop is not so loud that it scares little children and the bag is usually not damaged and so you can keep doing this again and again. Each kid gets to have bag.

B. Dry ice in a balloon. Just add a few pellets to a balloon and tie it up. Wait a few minutes and it will expand, and perhaps pop.

C. Smash a gummy bear [snake in Australia]. Make as slurry of dry ice and car antifreeze. Add a gummy bear. Take it out and smash it with a hammer. Aside: a technical discussion is here.

D. The cauldron. Simply add dry ice to some water and watch it "boil". This should actually lead to a good discussion of the difference between "bubbling" and "boiling".

Here is one compilation including a massive soap bubble by the "Crazy Russian Hacker". I did not do all of these!

In the USA I believe you can buy dry ice at some grocery stores. In Australia, it is harder; we had to go to a BOC Gas and Gear store in Brisbane and buy 1 kg of pellets for $10. They last about half a day before they completely sublimate. Pellets are easier to work with, but they don't last as long.

Postscript. The Youtube video of the LED lighting up when it is stuck in a lemon is a hoax. Because I saw it with my eyes I thought it was real. I am embarrassed; I really should have realised it could not be true. The key feature of a battery [electrochemical cell] is that the anode and the cathode have to be different materials so they have a different electrochemical potential.
But, I think making a real lemon battery would be cool. But it does require 3 to 4 lemons hooked up in series to produce the necessary minimum voltage to light the LED. I want to think about how this could be done in thermodynamics class to illustrate certain important concepts such as the chemical potential.

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